The present invention relates to an objective optical system for an optical pick-up which is used to write/readout data from/on an optical disc such as CD or DVD.
The optical pick-up is generally configured such that a laser beam emitted by a laser diode is converged on a data recording surface of the optical disc as a beam spot.
For example, in an optical data reproducing and/or recording device, a laser beam is emitted by a laser diode, as a diverging beam having an oval-shaped cross section. The diverging laser beam is then collimated by a collimating lens arranged in front of the laser diode. The collimated laser beam is incident on a beam shaping prism, which changes the oval cross section of the laser beam to a circular cross section. Then, using, for example, a mirror, the laser beam is reflected to proceed in a direction parallel with a radial direction of the optical disc. The laser beam is reflected again to proceed toward the data recording surface of the optical disc, by a mirror provided in a carriage. The carriage may be configured to be movable in a tracking direction, so that the laser beam is incident on the data recording surface perpendicularly via an objective optical system, which is also mounted on the carriage and converges the laser beam on the data recording surface of the optical disc.
An earlier objective optical system typically consisted of a single lens element, which is formed according to a plastic molding method. Therefore, a numerical aperture NA of the objective optical system was relatively small. Since a data density of the optical disc such as a CD is relatively small, the NA of the objective optical system was sufficient.
Recently, the data density has become relatively large, which requires a relatively small beam spot formed on the data recording surface of the optical disc. Accordingly, the numerical aperture is required to be 0.8 or more. The assignee has suggested, in Japanese Patent Application No. 2000-29879, an objective optical system consisting of a single-element glass lens having a very high NA.
When such a glass lens is formed using a mold, it is difficult to align the central axes of components of the mold respectively for forming both lens surfaces of the objective optical system. That is, in order to pick up a lens from the mold, the mold should be separated into two components. In order for allowing the mold to move inside a molding apparatus, clearances should be provided between the components of a mold, and between each mold and molding apparatus. Within such a clearance, the position/orientation of the molds is not restricted. As a result, it becomes difficult to align the central axes of both components of each mold, and the thus formed lens surfaces are decentered (i.e., the two surfaces are shifted relative to each other in a direction perpendicular to the optical axis of the objective lens). If the lens surfaces are decentered with each other, the performance of the lens in terms of the wavefront aberration is deteriorated. Therefore, it is important that the objective lens is designed so that the allowance of the decentering amount is made as large as possible, thereby the objective lens can be manufactured easily. In the above-described application, the objective lens is formed as a plano-convex lens so that the decentering allowance is very large, almost limitless.
When a plano-convex lens is formed to have a high NA, more than 0.8, coma cannot be compensated sufficiently for the abaxial light rays since one of the surfaces is a planar surface. Then, an area of the image side lens surface capable of maintaining a sufficient spot performance is limited. As a result, when the objective lens is mounted on a pick-up device or a fine actuator, the central axis of the incident beam and the optical axis of the objective lens should be aligned precisely. Therefore, the assembling efficiency is worsened. Further, if an alignment of the central axis of the incident beam and the optical axis of the objective lens is shifted due to some reason after the objective lens is fixed, the performance of the lens regarding the wavefront aberration is deteriorated excessively. Thus, allowance of an assembling error of the optical pick-up is very small, and hard to use.
The present invention is advantageous in that the above problems are solved. Further, although the objective lens is a single-element lens having a relatively large numerical aperture, it provides a sufficiently large image circle, thereby an assembling margin is relatively large. Further, according to an objective lens provided according to the invention, even though the surfaces are decentered within a predetermined allowance when the objective lens is manufactured with a mold, the performance thereof is not deteriorated. Therefore, the objective lens can be manufactured at a high yield ratio.
According to embodiments of the invention, there is provided a single-element objective lens for an optical pick-up, said objective lens converging substantially parallel light incident thereon onto a data recording surface of an optical recording medium. A first surface, which is a light incident side surface, of said objective lens is an aspherical surface having a positive power. A second surface, which is an optical recording medium side surface, of said objective lens is an aspherical surface having one of positive and negative power. Paraxial wavefront aberration is 0.07xcex rms or less when a decentering amount between said first and second surfaces is 2 xcexcm or less so as to converge the incident light substantially to a diffraction limit, a numerical aperture of said objective lens being 0.8 or more.
With this configuration, since the NA is 0.8 or greater, the objective lens can be used for an optical disc having a relatively high data density. Since the objective optical system consists of the single-element lens, a fine actuator conventionally used for holding a conventional single-element objective lens can be used for the high NA objective lens. Further, according to the objective lens configured as above, an image circle is enlarged. Therefore, even though the light beam, which consists of substantially parallel light rays, emitted by a light source unit of the optical pick-up device is inclined with respect to the optical axis of the objective lens, the performance of the lens in terms of the wavefront aberration will not be deteriorated significantly.
Optionally, the objective lens is made of glass material having a refractive index of 1.6 or more, with the objective lens being formed by molding.
Further, when an aspherical surface is expressed by equation:                               Fi          ⁡                      (            h            )                          =                ⁢                                            h              2                                      1              +                                                1                  -                                                            (                                              1                        +                        κ                                            )                                        ⁢                                                                  h                        2                                            /                                              r                        2                                                                                                                          +                                    A              4                        ⁢                          h              4                                +                                    A              6                        ⁢                          h              6                                +                                                ⁢                                            A              8                        ⁢                          h              8                                +                                    A              10                        ⁢                          h              10                                +                                    A              12                        ⁢                          h              12                                          
where, h represents a height of a point on the aspherical surface with respect to the optical axis, Fi(h) represents a SAG amount which is defined as a distance from a point on the aspherical surface to a plane tangential to the aspherical surface at the optical axis, i represents a surface number (i.e. F1(h) represents the shape of the light source module side, and F2(h) represents the shape of the optical disc side), r is a radius of curvature on the optical axis, k represents a conical coefficient, A4, A6, A8, A10 and A12 are aspherical coefficients for fourth, sixth, eighth, tenth and twelfth order terms,
then, each of the first and second surfaces are preferably formed to satisfy conditions below:
xe2x88x922xe2x89xa6dF1(h)/dhxe2x89xa6+2,
xe2x88x920.1xe2x89xa6dF2(h)/dhxe2x89xa6+0.1,
+0.5xe2x89xa6d2F1(h)/dh2xe2x89xa6+3.5,
and
xe2x88x920.2xe2x89xa6d2F2(h)/dh2xe2x89xa6+0.2.